This change improves copy_file_range(), sendfile(), splice(), openpty(),
closefrom(), close_range(), fadvise() and posix_fadvise() in addition to
writing tests that confirm things like errno and seeking behavior across
platforms. We now less aggressively polyfill behavior with some of these
functions when the platform support isn't available. Please see:
https://justine.lol/cosmopolitan/functions.html
